A typical vapor compression refrigeration system includes one or more evaporators disposed within a space to be cooled, a condenser disposed outside of the space to be cooled, a compressor for circulating a working fluid between the evaporator and the condenser, and expansion valves located between the condenser outlet and the inlet to each evaporator. The compressor compresses the working fluid in the form of vapor that is then circulated through the condenser where it is cooled and condensed to a liquid. The working fluid is then expanded through the expansion valve to reduce the pressure and to flash the working fluid into a liquid/vapor mixture. This mixture is then circulated through the evaporator.
In the evaporator, the working fluid absorbs heat from the space to be cooled and evaporates and is heated to become a superheated vapor. A superheated vapor is vapor at a temperature higher than the saturation temperature for a particular pressure. Prior vapor compression refrigeration systems have usually been designed to control the expansion valve to provide a large safety margin of superheat to avoid introducing liquid working fluid into the compressor. The introduction of liquid into the compressor may damage the compressor. The large safety margin has been required to accommodate, for example, variations in cooling load, ambient conditions at the condenser, system response times, and measurement inaccuracies.